1. Field of the invention
The present invention relates to an automotive transmission and more specifically to a transmission equipped with a 4-2 sequence valve which enables a 4-2 shift to assuredly take place in D-RANGE and which is responsive to a manual downshift command in a manner which ensures that the transmission is conditioned to produce second speed immediately before engine braking is enabled and thus avoid transmission interlock.
2. Description of the Prior Art
JP-A-62-62047 discloses a transmission arrangement which includes a 4-2 sequence valve. This valve is arranged to permit a 4-2 shift to directly take place in D-RANGE operation without the transmission being conditioned to produce 3rd speed. The valve is arranged such that in both 3rd and 4th speeds it acts as a switching valve which is responsive to the hydraulic pressure which is used to induce said speeds, in a manner which controls the draining of the servo chamber associated with the production of 4th speed.
In the case that a 4-2 shift is being executed, while the pressure which is supplied to the servo chamber which is associated with the production of 3rd speed, is above a predetermined level, the sequence valve does not switch states and the supply of line pressure to the servo chamber associated with the production of 4th speed is maintained thus maintaining 4th speed. Upon the pressure in the 3rd speed servo chamber dropping to the above mentioned low level, the sequence valve switches and assumes a condition wherein a drain port through which the pressure in 4th speed servo chamber can be exhausted.
However, this arrangement has suffered from the drawback wherein during a manually induced 4-2 downshift there is a delay before engine braking is rendered possible. That is to say, with the above type of transmission control arrangement engine braking wherein torque can be transmitted from the transmission output shaft back through the transmission to the engine, it is necessary to induce engagement of an overrunning clutch. In fact it is preferable to engage the overrunning clutch immediately following the gear selection which requires engine braking.
However, if the overrunning clutch and the band brake which is operated by the 4th speed servo are engaged simultaneously, a partial transmission interlock tends to occur and results in the generation of a substantial shift shock. Accordingly, it is necessary to delay the engagement of the overrunning clutch until such time as the 3rd speed pressure lowers sufficiently and the 4-2 sequence valve permits the pressure which maintains 4the speed to be drained. However, this results in the actual engagement of the overrunning clutch and the conditioning of the transmission to produce engine braking being undesirably delayed following the manual selection.